Anode current stabilization circuit x-ray tube having stabilizer electrode

ABSTRACT

A stabilization circuit for maintaining the anode current of an x-ray tube substantially constant is described including a first stabilization means in which a stabilization voltage signal proportional to changes in the anode current is applied to a stabilization electrode in such tube. A stabilization electrode is positioned behind the cathode filament with respect to the anode and collects a portion of the electrons emitted from the filament to reduce the number of electrons striking the anode. The amplitude of the stabilization voltage determines the amount of electrons collected by the stabilizer electrode and thereby maintains the anode current constant. A second stabilization means of slower operation gradually takes over from the first stabilization means and varies the cathode filament heating current in order to maintain the electron flow to the anode constant in response to a control signal produced by a differential amplifier comparator means which is proportional to the stabilization voltage applied to the stabilizer electrode. Light signal coupling is employed between the x-ray tube anode current monitor and the stabilizer electrode power supply, as well as between such power supply and the differential amplifier for voltage insulation purposes.

United States Patent [191 Fulton et al. Jan. 1, 1974 ANODE CURRENTSTABILIZATION [57] ABSTRACT CIRCUIT XRAY TUBE HAVING A stabilizationcircuit for maintaining the anode cur- STABILIZER ELECTRODE rent of anx-ray tube substantially constant is de- [75] Inventors: George LFulton, Chicago; Robert scribed including a first stabilization means inwhich a Gaga. Elmhurst; Zed J. Aflee, stabilization voltage signalproportional to changes in Oak Brook a" CH the anode current is appliedto a stabilization electrode in such tube. A stabilization electrode isposi- Assigneer Pick" Corporation, Cklveland, tioned behind the cathodefilament with respect to the Ohio anode and collects a portion of theelectrons emitted [22] Filed: 18, 1972 from the filament to reduce thenumber of electrons striking the anode. The amplitude of thestabilization PP ,646 voltage determines the amount of electronscollected by the stabilizer electrode and thereby maintains the 52 us.Cl. 250/404, 250/409 anode current Constant A mild Stabilization means51 lm. Cl H05g 1/34 of Slower Operation gradually takes from the first58 Field of Search 250/95, 97, 99, 103 Stabilization means and variesthe cathode filament heating current in order to maintain the electronflow [56] I References Cited to the anode constant in response to acontrol signal UNITED STATES PATENTS produced by a differentialamplifier comparator means which is proportional to the stabilizationvolt- 2,767,327 l0/l956 Helterline,Jr. 250/97 age applied to theStabilizer electrode Lightsignal 3,633,029 l/l972 Duffy, Jr 250/99Primary Examiner-James W. Lawrence Assistant E.\'aminer-B. C. AndersonAttorney-Stephen W. Blore et al.

l4 [0 f 30 24 l l8 EXPOSURE UGHT CONTROL ELECTRODE PULSER PULSE POWERSUPPLY JJ20 26 J X-RAY TUBE coupling is employed between the x-ray tubeanode current monitor and the stabilizer electrode power supply, as wellas between such power supply and the differential amplifier for voltageinsulation purposes.

10 Claims, 2 Drawing Figures FAST STAB] LIZE LIGHT SIGNAL SIGNAL POWERSUPPLY i s AMPLIFlER LNQ'VATLIGHT SIGNAL 38 46 i I" w CATHODE ANODE iVOLTS vou's I LIGHT 48 5O 52 22 i 5.8G- CATHODE FILAMENT HIGH VOLTAGEmgvtwDlFFERENTl/Rl. v CURRENT HEATER TRANSFORMER i AMPUF'ER STABILIZERTRANSFORMER J i 43 56 HIGH LOW ANODE L REEV corsa r iURRENT CURRENT n wm w 54 LIGHT MONITOR SIG. 35

ANODE CURRENT STABILIZATION CIRCUIT X-RAY THEE HAVING STABILIZERELECTRODE BACKGROUND OF THE INVENTION The subject matter of the presentinvention relates generally to the stabilization of electron dischargecurrent flowing between the cathode and anode of an x-ray tube or otherhigh voltage electron discharge device. In particular, the inventionrelates to an electrical circuit for operating an x-ray tube having astabilizer electrode of the type shown in pending US. Pat. applicationSer. No. 229,136 filed Feb. 24, 1972, by Zed J. Atlee and entitled X-RAYTUBE WITH IMPROVED CON- TROL ELECTRODE ARRANGEMENT, in order to maintainthe anode current of such tube substantially constant. This is achievedby applying a stabilizing voltage signal to the stabilizer electrodewhich is proportional to any variation in the anode current, suchstabilizing voltage being of a polarity and amplitude which causes areduction in such anode current variations. The stabilization circuit ofthe present invention is especially useful in pulsed x-ray apparatus,such as cineradiography, and may be employed for an x-ray tube havingtwo filament cathodes, one of which is for low current fluoroscopy andthe other for high current spot film radiography.

Previous x-ray tube stabilization circuits such as that shown in U.S.Pat. No. 2,617,045 of M. R. Coe and U5. Pat. No. 2,810,838 of C. W.Clapp et al stabilized the cathode to anode discharge current of thex-ray tube by adjusting the heating current flowing through the cathodefilament to change the electron emission current of the cathode. Thistechnique is satisfactory for tungsten cathode filaments which areheated to an extremely high temperature during normal operation so thatheating and cooling of the filament to adjust the emission current maybe accomplished rapidly by changing the heating current. However, thistype of stabilization is too slow for some application of x-ray tubesemploying thoriated tungsten filaments because they are operated at amuch lower temperature than pure tungsten filaments, and therefore coolat a slower rate. Thus, for example, when making rapid exposures lessthan about a second duration, x-ray tubes employing thoriated tungstenfilaments cannot be operated using prior stabilization circuits withoutsome compromise in the degree of stabilization obtained because they donot stabilize the anode current fast enough.

The stabilization circuit of the present invention overcomes thesedisadvantages by providing a separate stabilization electrode in thex-ray tube behind the cathode filament with respect to the anode andapplying a stabilizing voltage signalto such stabilizer electrode whichis proportional to variations in the anode current of the x-ray tube.The stabilizer electrode responds immediately and collects a portion ofthe emitted electrons to reduce the electron discharge current to theanode with the amount of such collected portion changing in accordancewith the amplitude of the stabilizing voltage applied thereto, andthereby maintains the anode current substantially constant. This newstabilization means operates substantially instantaneously since it doesnot depend upon the heating and cooling rate of the cathode filament.

In addition, the stabilization circuit of the present invention may alsoemploy a second stabilization means which is activated by a controlsignal porportional to 2. the stabilizing voltage output signal of thefirst stabilization means described above and operates by varying thecathode filament heating current in a similar manner to that of theabove-cited references. The second stabilization means gradually takesover stabilization of the anode current from the first stabilizationmeans and reduces the stabilizing voltage signal. This provides a moreefficient operation and prevents undue heating of the stabilizerelectrode clue to electron discharge current. Light signal coupling isemployed between an anode current monitor in the high voltagetransformer circuit and the input of the first stabilizer means toproduce the stabilizing voltage signal without the need for high voltageinsulation. A similar light coupling is employed between the output ofthe first stabilization means and the input of a differential amplifierin the second stabilization means in order to apply a control signal tothe second means which is proportional to the stabilizing voltage signalof the first means.

It is, therefore, one subject of the present invention to provide animproved stabilization circuit of fast re spouse for maintaining theanode current of an x-ray tube or other electron discharge devicesubstantially constant.

Another object of the inventin is to provide such a stabilizationcircuit, including a first stabilization means, in which a stabilizingvoltage signal proportionalto changes in the anode current is applied toa stabilizer electrode within the x-ray tube in order to collect aportion of the emitted electrons and reduce the electron dischargecurrent reaching the anode.

A further object of the invention is to provide such a stabilizationcircuit of more efficient operation in which a second stabilizationmeans of slower response is employed which gradually takes overstabilization from the first stabilization means by adjusting thecathode filament heating current in order to maintain the anode currentsubstantially constant in accordance with a control signal proportionalto the stabilizing voltage of the first stabilization means.

An additional object of the invention is to provide such a stabilizationcircuit in which light signal coupling is employed in order to providehigh voltage isolation and fast response.

An additional object is to provide such a stabilization circuit suitablefor pulsed x-ray apparatus using x-ray tubes having filament cathodesmade of thoriated tungsten in order to quickly stabilize the cathode toanode discharge current of such tubes.

BRIEF DESCRIPTION OF DRAWINGS DESCRIPTION OF PREFERRED EMBODIMENT Asshown in FIG. 1, one embodiment of the stabilization circuit of thepresent invention includes an x-ray tube ll) or other electron dischargedevice, having an anode l2 and a thermionic cathode 14 which ordinarilyis a filament cathode of thoriated tungsten but could be an indirectlyheated cathode having a separate heating filament. A stabilizerelectrode 16 is provided within the evacuated tube envelope outside thefocusing field of the cathode and preferably in a position behind thecathode 14 with respect to the anode 12. This position of the stabilizerelectrode is necessary to prevent the stabilizing signal voltagesapplied thereto from defocusing the electron beam and changing the focalspot of such beam on the anode 12. Thus, the source of the xraysprovided by such focal spot does not vary in size with changes instabilizing voltage. In addition, a control electrode 18 is providedbetween the cathode 14 and anode 12 to enable the x-ray tube to bepulsed on and off. The control electrode 18 is shown as a grid, butwhich is actually a cathode cup type focusing electrode as shown incopending U. S. Pat. application Ser. No. 229,136, filed Feb. 24, 19-72,by Zed J. Atlee entitled: X-RAY TUBE WITH IMPROVED CONTROL ELECTRODEARRANGEMENT. This control electrode is quiescently biased to cut off thecathode 14 and is momentarily pulsed to a more positive voltage to allowelectron discharge current to flow between the cathode l4 and the anode12. This enables the cathode filament 14 to be heated continuously evenwhile the tube is cut off and thereby provides the tube with a fasteremission response when it is pulsed on, which is extremely useful forcineradiography requiring rapid pulses of high current. In addition, asecond cathode filament 20 may be provided in the tube to provide acontinuous electron discharge of low current during fluoroscopy so thatthe patient is exposed to low x-ray radiation levels.

A high voltage transformer 22 is connected at it positive output ofabout +50 kilovolts to the anode l2 and has its negative output ofabout-O kilovolts connected through a control electrode power supplycircuit 24 to a common lead 26 of both cathode filaments 14 and 20.Another output of the control electrode power supply 24 is connectedthrough lead 28 to the control electrode 18 to apply positive voltagepulses of about 53.5 kilovolts quiescent value and 50.0 kilovolts peakvalue to the control electrode in order to switch the x-ray tubemomentarily on from its normal off state. These pulses-are produced byan exposure pulser circuit 30 which produces a light pulse 32 that isdetected by a photoelectric detector in the power supply 24. Thus, thedurationof the light pulse 32 determines the width of the voltage pulsesproduced on output 28.

The stabilizer electrode 16 is connected through a lead 34 to the outputof a stabilizer electrode power supply and amplifier 36 whose input is alight signal 38 produced by an anode current monitor 40. The anodecurrent monitor 40 is connected to the high voltage transformer 26 so itdetects changes in the electron discharge current flowing from one ofthe cathodes l4 and to the anode 12 of the x-ray tube and produces acorresponding light signal output 38 whose light intensity correspondsto the magnitude of the anode current variations. This light signal 38is transmitted to a photoelectric detector in the stabilizer electrodepower supply 36 which produces a corresponding stabilizing voltage atoutput 34. The stabilizing voltage is proportional to the intensity ofthe light signal 38 and is of a polarity to oppose the change in anodecurrent detected by monitor circuit 40. Thus, when the current in theanode 12 decreases, a negative going stabilizing voltage signal isapplied to the stabilizer electrode 16 so that fewer emitted electronsare collected by the stabilizer electrode and more emitted electrons areattracted to the anode 12, thereby increasing the anode current to itsdesired value. Conversely when the anode current sensed by monitor 40increases a positive going stabilizing voltage is produced on lead 34which causes more of the emitted electrons to be attracted to thestabilizer electrode 16. This reduces the number of emitted electronstransmitted to anode 12 and thereby decreases the anode current to itsdesired value. Thus, the stabilizer electrode 16, the power supply andamplifier 36, and the anode current monitor 40 provide a firststabilization means which acts in the manner of the negative feedback tomaintain the anode current of the x-ray tube substantially constant.This first stabilization means has the advantage that it has asubstantially instantaneous response.

In addition to the first stabilization means, a second stabilizationmeans of slower response which gradually takes over from the firstmeans, may also be employed for more efficient operation. The secondstabilization means includes a differential amplifier 42 connected as acomparator means having one input 43 connected to a DC. referencevoltage on the movable contact of a potentiometer 44 whose end terminalsare connected, respectively, to positive and negative D.C. supplyvoltages. The other input of the differential amplifier is a lightsignal 46 transmitted from the output of the stabilizer electrode powersupply 36 and having a light intensity corresponding to the value of thestabilizer voltage produced on output 34 of such power supply. The lightsignal 46 is transmitted to a photoelectric detector in the differentialamplifier 40 which produces a corresponding input voltage signal that iscompared with the reference voltage on input 43 and produces a controlsignal across the output terminals 48 of the differential amplifier. Theoutput 48 of differential amplifier 42 is applied to the input of acathode current stabilizer circuit 50 whose output is connected to afilament heater transformer 52 in order tovary the heating currenttransmitted to the cathodes l4 and 20 of the x-ray tube. Thus, thecathode current stabilizer circuit 50 may be of the type shown in the U.S. Pat. No. 2,617,045 of Coe or in U. S. Pat. No. 2,810,838 of Clapp etal. which changes the impedance in series with the primary winding ofthe heater transformer in order to vary the heater current in thesecondary winding of such transformer transmitted to the cathodefilaments 14 and 20.

This second stabilization means 42, 50 and 52 is slower operating thanthe first means, and also acts to maintain the anode current of the,x-ray tube substantially constant and gradually takes overstabilization from such first means. Thus, when the anode currentincreases greater than the desired value, a positive going stabilizingvoltage is produced at the output 34 of the stabilizer electrode powersupply 36 and a corresponding light signal 46 is transmitted to thedifferential amplifier 42 which compares it with the reference voltage43 and produces a control signal at the output 48 of such amplifier.This control signal decreases the heater current flowing in filament 14to lower the temperature of the filament and reduces the electronemission of such filament. As a result, the anode current decreases andthe light signal 38 of monitor 40 reduces in intensity which in turnlowers the stabilizing voltage 34 and lowers the intensity of lightsignal 46. This continues until, the anode current of the x-ray tube isstabilized at the desired value at which time the control signal output48 of the differential amplifier 40 reduces to zero.

It should be noted that a selector switch 54 may be connected in serieswith an AC. line voltage source 56 for selectively. applying such linevoltage to the primary winding of the heater transformer of either thehigh current filament 14 of the low current filament 20 for selectiveenergization thereof. In addition, it is possible to use the switch 54to selectively energize the filament merely by shorting a high seriesimpedance to increase the heating current therethrough to an emissionlevel, while maintaining the heating currents in both filaments at alower standby current level in order to increase the speed of responsewhen switching between the filaments.

The stabilizer apparatus of FIG. 1 may employ the electrical circuit ofFIG. 2 which is hereafter described, using the same reference numeralsto indicate the dashed line boxes in FIG. 2 which corresponds to theblocks of FIG. 1 for easier understanding of the circuit. Thus, the highvoltage transformer 22 is a three phase transformer including threesecondary coils 58 which are connected through diodes 60 to a commonanode voltage lead 62 for applying a positive voltage of about +50kilovolts to the anode 12 of the x-ray tube 10. A current limiter diodetube 64 is connected in series between the output terminal 62 of thehigh voltage trans-' former and the anode 12 of the x-ray tube and afilter including a resistor 66 in parallel with a capacitor 68 isconnected from the anode of such limiter tube to ground. Three othersecondary coils 70 on the high voltage transformer are connected throughother rectifier diodes 72 to a common cathode voltage lead 74 to supplya negative voltage of about 50 kilovolts to the common lead 26 atcathodes 14 and 20 of the x-ray tube through the controlelectrode powersupply 24. Another current limiter diode tube 76 is connected in serieswith the output 74 of the high voltage transformer. A filter including aresistor 75 and a parallel capacitor 77 are connected between thecathode of diode 76 and the ground. The anode of diode 76 is connectedto a common point 78 in the control electrode power supply to apply anegative voltage of about 50 kilovolts thereto. This 50 kilovolts servesa reference voltage for the control electrode 18, the cathodes 14 andand the stabilizer electrode 16, and is transmitted through a zennerdiode 80 to the secondary windings of a control electrode power supplytransformer 82.

The control electrode power supply 24 includes a first secondary winding84 on transofrmer 82 which is connected across a full wave rectifierbridge 86 to produce a positive D.C. voltage of about +l,750 volts onthe right side of the bridge rectifier which is applied to the upperterminal of a resistor 88 and produces a negative D.C. voltage of about1 ,750 volts on the left side of the bridge which is applied to thelower terminal of resistor 88. A capacitor 90 is connected in parallelwith resistor 88 to provide a ripple filter and the upper terminal ofsuch filter is connected through a current limiter resistor 92 to thecommon terminal 26 of the cathode filaments l4 and 20 of the x-ray tube.The control electrode 18 is connected by lead 28 through a pair ofseries resistors 94 and 96 to the common connection 78 so that anegative voltage of about -50 kilovolts is applied to such controlelectrode. A quiescent voltage of about 47 kilovolts is applied to thecathode 14 which is the sum of the +3 kilovolts produced across bridge86 and the 50 kilovolts applied to such bridge through a lead 98. Thus,the x-ray tube is quiescently reversed biased approximately 3,000 voltsto cut off such tube.

A pair of vacuum tubes 100 and 102 which may be of the pentode type areconnected in parallel with their cathodes connected in common to thelead 98 and their anodes connected in common to the current limiterresistor 92. The control grids of tubes 100 and 102 are connectedthrough coupling resistors 104 and 106, respectively, to a source ofpositive voltage switching pulses which momentarily render such tubesconducting from their quiescent nonconducting state. This effectivelyshort circuits the control electrode power supply so that the entire3,000 volts is dissipated in current limiter resistor 92. As a result,the cathode voltage on lead 26 momentarily reduces to 50kilovolts sothat it equals the control electrode voltage on lead 28, therebyremoving the reverse bias on the x-ray tube and rendering such tubeconducting. As a result, electrons emitted from the cathode 14 arecaused to bombard the anode 12 to produce an x-ray pulse, which may beemployed in cineradiography. to record an x-ray image on film. Thus, atthis time, the control electrode 18 and the cathode 14 of the x-ray tubeare maintained at the same potential by a reference resistor 108 anddiode 110 connected in series between leads 26 and 28.

An overvoltage protection resistor 112 in series with a zener diode 114are connected across the vacuum tubes 100 and 102 to prevent damage tosuch tubes when they are in a nonconducting state. Thus, whenovervoltage occurs, the zener diode 114 conducts and transmits currentthrough resistor 112 to ground through another resistor 116 connectedthereto. An A.C. coupling capacitor 118 is connected between the commonlead 98 and the junction of resistors 94 and 96 to handle transientvoltages. The heater voltage applied to the filaments of the vacuumtubes and 102 and diode 76 is supplied by another secondary winding 120on the transformer 82 which is referenced to the common lead 98 and isconnected to such filaments by lead 121. In addition, the middle gridsof tubes 100 and 102 are connected through resistors 1122 and 124. tothe common point 78, with the upper grids connected to the cathodes ofsuch tubes.

The exposure pulser circuit 30 includes a first NPN type switchingtransistor 126 whose base is connected through a coupling resistor 128to an input terminal 130 to which is applied a positive rectangularpulse having a width corresponding to the desired x-ray exposure time.This pulse switches transistor 126 conducting from a quiescentnonconducting state to produce a negative pulse on its collector. Thecollector of transistor 126 is connected through a load resistor 132 toa source of D.C. supply voltage of about +20 volts, while the emitter ofsuch transistor is connected directly to ground and its base isconnected to ground through a bias resistor 134. The negative goingoutput pulse on the collector of transistor 126 is applied to the baseof a second NPN switching transistor to cause it to switch to anonconducting state from its quiescent conducting state. The collectorof transistor 136 is connected through a load resistor 138 to the +20volts D.C. supply voltage and has its emitter grounded. When transistor136 switches off it produces a positive going pulse on its collectorwhich is applied to the base of a third NPN switching transistor 140 toswitch it to a conducting state from its normally nonconducting state.The emitter of transistor 140 is grounded, while its collector isconnected through a light emitting diode 142 and a load resistor 144 tothe volts D.C. supply. When transistor 140 is rendered conducting acurrent pulse is transmitted through the light emitting diode 142causing such diode to emit a light pulse 32 ofa duration correspondingto the width of the x-ray exposure pulse ap plied to the input terminal130 of the pulser 30.

This light pulse 32 is transmitted to a light sensitive transistor 146connected as a diode with its base shorted to its emitter, in thecontrol electrode power supply 24. The light pulse renders thetransistor 146 conducting so that it effectively short circuits avoltage divider resistor 148 connected in parallel between the emitterand collector of such transistor. This causes more current to flowthrough a second voltage divider resistor 150 connected between the baseand emitter of a normally nonconducting NPN transistor 152 to switchsuch transistor to a'conducting state. The common terminal of theemitter of transistor 152 and the voltage divider resistor 150 isconnected to the common lead 98 so that a potential of 50 kilovolts isapplied thereto. For this reason, the light coupling 32 is employed toelectrically insulate the low-voltage pulser 38 from the high voltage ofthe control electrode power supply 24. The collector of transistor 152is connected through a load resistor 154 to a +50 volt D.C. supplyvoltage having an actual value of about -49,95O volts produced at theoutput of a rectifier bridge 156 having its input connected across athird secondary winding 158 on transformer 82. Thus, the other output ofthe bridge is reference to -50 kilovolts on the common lead 98 which isadded to the +50 volts produced across the bridge. When transistor 152is switched conducting, it produces a negative going pulse which istransmitted through a coupling resistor 160 to the base of an NPN outputtransistor 162 in order to switch such output transistor to anonconducting state from its normally quiescent conducting state. Theoutput transistor 162 has its emitter connected to lead 98 and has itcollector connected through a load resistor 164 to the +50 volts supplyvoltage provided at the output of bridge 156. It. should be noted that aresistor 166 in parallel with the capacitor 168 are connected across theoutput of the bridge 156 to provide a ripple filter which smooths the 50volts D.C. supply voltage The positive going exposure control pulsesproduced on the collector of the output transistor 162 are transmittedthrough a protection diode 170 to the grids of the tubes 100 and 102 inorder to switch such tubes into a momentarily conducting state. In thismanner, the x-ray tube is provided with a pulsed operation since thereverse bias between the control electrode 18 and the cathode 14 ismomentarily removed to enable electrons to flow from such cathode to theanode 12, and thereby produce an x-ray pulse.

As stated previously with regard to FIG. 1, in order to maintain thecathode to anode discharge current of the x-ray tube substantiallyconstant during rapid exposure times, a stabilizing voltage istransmitted from the output of the stabilizing electrode power supplyand amplifier 36 through lead 34 to the stabilizer electrode 16. Thiscauses the stabilizer electrode to collect a portion of the electronsemitted by the cathode 14 and thereby control the amount of dischargecurrent reach ing the anode 14. The stabilizer electrode power supply 36includes a secondary winding 172 which may be provided on thetransformer 82 and a bridge rectifier 174 connected across such windingto produce a rectified D.C. voltage of about 50 volts at the outputterminals of such bridge. A ripple filter, including a capacitor 176 inparallel with resistor 178, is connected across the bridge. The lowerterminal of the rectifier bridge 174 is connected to the common lead 98so that it is also referenced to 50 kilovolts. The light signal 38produced by the monitor 40 is received by a photosensitive transistor180 of NPN type connected as a diode in the stabilizer electrode powersupply 36. The emitter and collector of the phototransistor areconnected in parallel with a voltage divider resistor 182. The voltagedivider resistor 182 is connected through a current limiting resistor184 to the upper terminal of bridge 94 and is connected in series withanother voltage divider resistor 186 which also acts as the base biasresistor of an NPN switching transistor 188. Thus, when a light signal38 is received, the emitter to collector impedance of transistor 180reduces so that the current through the bias resistor 186 increased andproduces a positive going signal on the base of transistor 188 whichacts as a first inverter amplifier. The collector of transistor 188 isconnected through a load resistor 190 to the positive D.C. supplyvoltage, while its emitter is connected to the 50 kilovolts D.C.reference voltage on lead 98. Thus, transistor 188 inverts the positivesignal and transmits a negative signal from its collector whose voltageamplitude depends on the intensity of the light signal 38.

The negative going signal produced on the collector of transistor 188 istransmitted through a peaking circuit including a coupling resistor 192in parallel with a capacitor 194, to the base of another NPN transistor196 connected as a second inverter amplifier. The base of transistor 196is connected to the negative D.C. supply voltage through a base biasresistor 198 while its emitter is directly connected thereto and itscollector is connected to the positive D.C. supply voltage through aload resistor 200. The negative going signal is amplified and invertedto produce a positive going signal on the collector of transistor 196which is then transmitted through another peaking circuit including acoupling resistor 202 in parallel with a capacitor 204. These peakingcircuits increase the slope of the leading edge of the signal to provideit with a faster rise time. The positive going signal is transmitted tothe base of a third inverter amplifier transistor 205 which produces anegative signal on its collector. The base of transistor 205 isconnected to the negative D.C. power supply through a bias resistor 206while its emitter is directly connected thereto, and its collector isconnected to the positive D.C. power supply through a load resistor 207.A fourth inverter amplifier transistor 208 having its base connected tothe collector of transistor 205 inverts the negative signal on its baseto produce a positive output signal on its collector. This transistor208 is also provided with a base bias resistor 209 and a collector loadresistor 210.

The positive going output signal of transistor 208 provides thestabilizing voltage produced at the output of the stabilizer electrodepower supply 36, and is transmitted through a light emitting diode 212and an overvoltage protection diode to the stabilizer electrode 16. Thispositive stabilizing voltage causes a portion of the electrons emittedby the cathode 14 to be collected by the stabilizer electode whichreduces the discharge current flowing in the anode 12 of the x-ray tube,and thereby tends to maintain such anode current substantially constant.The stabilizing voltage produced on the lead 34 varies between zero and+50 volts depending upon the brightness of the light signal 38 emittedby a light emitting diode 216 in the monitor 40. The light emittingdiode 216 has its cathode grounded and its anode connected through aselector switch 218 in series with one of a plurality of resistors 220of different value which are selectively connected in parallel with alimiter resistor 222. The common terminal of the resistors 220 andresistor 222 is connected at a common terminal 224 to both the cathodeand anode windings 70 and 58 of the high voltage transformer 22. Thus,the sum of the anode and cathode current of the x-ray tube istransmitted to ground through the light emitting diode 216 and a shuntresistor 226 connected in parallel therewith so that the diode 216 emitslight only when the total current exceeds a selected value of, forexample, about 50 milliamperes. Thus, the light signal 38 emitted bydiode 216 has a brightness which is proportional to the anode currentflowing in the x-ray tube 10. 1

The differential amplifier 42 of the second stabilizer means includes aphtosensitive transistor 228 of NPN type which is connected as a diodeat one input of such differential amplifier. The phototransistor 228 ispositioned to receive the light signal 46 transmitted from the lightemitting diode 212 of the stabilizer electrode power supply andamplifier 36. The other input of the differential amplifier is a DC.reference voltage on the movable contact 43 of potentiometer 44 which isconnected to the base of another NPN transistor 230 connected as aninverter amplifier on one side of the differential amplifier. A secondinverter amplifier transistor 232, similar to transistor 230, isprovided on the other side of the differential amplifier and has itsbase connected to the emitter of phototransistor 228. A bias resistor234 is connected between the emitter of transistor 232 and its base.When light signal 46 is produced, the emitter to collector impedance ofphototransistor 222 reduces, so that the current through bias resistor234 increases, thereby producing a positive voltage on the base oftransistor 232. This produces a differential output signal on outputconductors 48 of the differential amplifier when such positive voltageexceeds the reference voltage applied to the base of transistor 230 inthe manner of a conventional differential amplifier.

Transistors 230 and 232 are connected through emitter biase resistors236 and 238 in common to a source of negative D. C. supply voltageprovided at the lower terminal of a diode bridge rectifier 240 whoseinput terminals are connected across a secondary winding 242 ontransformer 82. The positive output terminal of the bridge 240 isconnected through load resistors 244 and 246, respectively, to thecollectors of transistors 230 and 232 via additional load resistors 248and 252. Load resistor 248 may be variable in order to balance theoutput voltages of the differential amplifier so that it produces nooutput signal unless a light signal is received by phototransistor 228.The collectors of transistors 230 and 232, respectively, are connectedto the bases of second inverter amplifier transistors 252 and 254.Transistors 252 and 254 have their emitters connected through zenerdiodes 256 and 258 to the positive D.C. supply and have the collectorsconnected through load resistors 260 and 262 to the negative D.C. supplyvoltage since these are PNP transistors. A pair of third inverteramplifier transistors 264 and 266 are provided with their basesconnected to the collectors of transistors 252 and 254, respectively,and their collectors connected to the output terminals 48 of thedifferential amplifier at load resistors 244 and 246.

The output signal of the differential amplifier 42 produced across leads48 upon receipt of the light signal 46 is applied to the cathode currentstabilizer 50 which varies an impedance in series with the common leadof a pair of primary windings 268 and 270 of the filament heatertransformer 52 in a similar manner to that shown in U. S. Pat. No.2,810,838 of Clapp et al., or in U. S. Pat. No. 2,617,045 of Coe. Thesecondary windings 272 and 274 of the transformer 52 are connected,respectively, to the cathode filaments l4 and 20 of the x-ray tube atterminals 276 and 278, respectively. Thus, in the shown position ofswitch 54, the stabilizer 50 changes the heating current offilament 14in order to vary the electron emission of such filament and therebymaintain the electron discharge current in anode 12 substantiallyconstant. As stated above, this second stabilizer means 42, 50 and 52 isslower than the first stabilizer means 16, 36, and 40 and graduallytakes over stabilization from such first means. This provides a moreefficient operation since the second stabilizer means reduces the totalelectron current emitted from the cathode 14 while the first stabilizermeans does not, and thereby reduces power dissipation and heating of thestabilizer electrode 16.

It will be obvious to those having ordinary skill in the art that manychanges may be made in the details of the above-described preferredembodiment of the present invention. For example, the two filaments 14and 20 of the x-ray tube can both be operated without any pulsed controlelectrode or a second control electrode may be provided for the filament20' in order to selectively bombard the anode with one of suchfilaments. In addition, for dual filament stabilization, a secondstabilizer electrode can be employed behind filament 20 which isconnected like stabilizer electrode 16. Also, it should be noted thatsince circuits 22, 24', 36 and 52 operate at a high voltage betweenabout 50 kilovolts and ISO kilovolts, they may be provided in an oilfilled tank for insulation purposes. Therefore, the scope of the presentinvention should only be determined by the following claims.

We claim:

1. An electron discharge current stabilization apparatus comprising;

an electron discharge tube having an anode, at least one thermioniccathode and a stabilizer electrode; power supply means for applying highvoltage between said anode and cathode, and for applying a heatingcurrent to said cathode to cause electrons to be emitted from saidcathode and to bombard said anode as an electron discharge current; andstabilization means including a first meats separate from saidstabilizer electrode for detecting the amount of the electron dischargecurrent flowing in said anode, said first means applying a stabilizingsignal voltage to said stabilzer electrode to cause a portion of saidelectrons to be diverted from said, anode to said stabilizer electrodedepending upon the value of said stabilizing voltage, and said firstmeans varying said stabilizing voltage in accordance with any changes inthe anode current in a manner which maintains said anode currentsubstantially constant. 2. A stabilization apparatus in accordance withclaim 1 in which the stabilization means also includes a second meansfor detecting the amount of electron current flowing in said stabilizerelectrode and producing a control signal corresponding thereto, andcontrol means for varying the cathode heating current to change theelectron emission of said cathode in accordance with said control signalin a manner which maintains said anode current substantially constant.

3. A stabilization apparatus in accordance with claim 2 in which saidsecond means is coupled to said first means by light coupling means.

4. A stabilization apparatus in accordance with claim 3 in which saidlight coupling means includes an electrical light source in said firstmeans which emits a light signal corresponding to said control signal,and a photoelectric detector in said second means for converting saidlight signal into said control signal.

5. A stabilization apparatus in accordance with claim 4 in which thesecond means includes a comparator means having one input connected tosaid photoelectric detector and another input connected to a DC.reference voltage, and having its output connected to said controlmeans.

6. A stabilization apparatus in accordance with claim 5 in which thetube is an x-ray tube, the power supply includes a filament transformerhaving its secondary winding connected to a filament cathode insaid'x-ray tube and having its primary winding connected to said controlmeans.

7. An x-ray apparatus comprising: an x-ray tube having an anode, atleast one thermionic cathode and a stabilizer electrode positionedbehind the electron emitter of said cathode with respect to said anode;power supply means for applying high voltage between said anode andcathode, and for applying a heating current to said cathode to causeelectrons to be emitted from said cathode and to bombard said anode asan electron discharge current; and

stabilization means including a first means separate from saidstabilizer electrode for detecting the amount of the electron dischargecurrent flowing in said anode, said first means applying a stabilizingsignal voltage to said stabilizer electrode to cause a portion of saidelectrons to be diverted from said anode to said stabilizer electrodedepending upon the value of said stabilizing voltage and said firstmeans varying said stabilizing voltage in accordance with any changes inthe anode current in a manner which maintains said anode currentsubstantially constant.

8. An x-ray apparatus in accordance with claim 7 in which the x-ray tubeincludes a focusing cup type control electrode separate from saidstabilizer electrode and connected to a pulser means for applying pulsesto said control electrode of sufficient voltage to change the bias onthe x-ray tube to a conducting state from a quiescent nonconductingstate.

9. An x-ray apparatus in accordance with claim 7 in which the firstmeans is coupled to a high voltage transformer of the power supply meansby a light coupling means for transmitting a light signal correspondingto said stabilizing signal from said power supply means to said firstmeans of said stabilization means.

10. An x-ray apparatus in accordance with claim 7 in which thestabilization means also includes a second means for detecting theamount of current flowing in the stabilizer electrode and for varyingthe cathode heating current of the x-ray tube to maintain its anodecurrent substantially constant, said first means operating faster thansaid second -means, and said second means being connected so that itgradually takes over stabilization of the anode current and reduces thestabilizing signal voltage so as to prevent electron heating of thestabilizer electrode.

Patent No. $7 3, Dated January 1, 1974 Inventor) George L. Fulton;Robert M. Gager; and Zed J. Atlee It is certified Lhat error appears inthe above-identified patent add that said Letters Patent are herebycorrected as shown below:

Page 1, the title should be corrected to read as follows:

ANODE CURRENT STABILIZATION CIRCUIT FOR X-RAY TUBE HAVING STABILI ZERELECTRODE Column 1, the title should be corrected to read as follows:

ANODE CURRENT STABILIZATION CIRCUIT FOR X-RAY TUBE HAVING STABILIZERELECTRODE I Column 5, line S2,"transofrmer" shouldbec --tra n sformer--;

Column 8, line 22, "increased" should be "increases- Column 10, claim1', line 60, "meats" should be -rmeans--.

oigned and sealed this 16th day of April 19m.

(SEAL) Attesb:

EDWARD 1=I.FLETCHER,JR. I I I c, 'I IARSEMLL DANN I Attesting; Officer ICommissioner of Patents r I I m J "ORM Inc-1050410039)

1. An electron discharge current stabilization apparatus comprising; anelectron discharge tube having an anode, at least one thermionic cathodeand a stabilizer electrode; power supply means for applying high voltagebetween said anode and cathode, and for applying a heating current tosaid cathode to cause electrons to be emitted from said cathode and tobombard said anode as an electron discharge current; and stabilizationmeans including a first means separate from said stabilizer electrodefor detecting the amount of the electron discharge current flowing insaid anode, said first means applying a stabilizing signal voltage tosaid stabilizer electrode to cause a portion of said electrons to bediverted from said anode to said stabilizer electrode depending upon thevalue of said stabilizing voltage, and said first means varying saidstabilizing voltage in accordance with any changes in the anode currentin a manner which maintains said anode current substantially constant.2. A stabilization apparatus in accordance with claim 1 in which thestabilization means also includes a second means for detecting theamount of electron current flowing in said stabilizer electrode andproducing a control signal corresponding thereto, and control means forvarying the cathode heating current to change the electron emission ofsaid cathode in accordance with said control signal in a manner whichmaintains said anode current substantially constant.
 3. A stabilizationapparatus in accordance with claim 2 in which said second means iscoupled to said first means by light coupling means.
 4. A stabilizationapparatus in accordance with claim 3 in which said light coupling meansincludes an electrical light source in said first means which emits alight signal corresponding to said control signal, and a photoelectricdetector in said second means for converting said light signal into saidcontrol signal.
 5. A stabilization apparatus in accordance with claim 4in which the second means includes a comparator means having one inputconnected to said photoelectric detector and another input connected toa D.C. reference voltage, and having its output connected to saidcontrol means.
 6. A stabilization apparatus in accordance with claim 5in which the tube is an x-ray tube, the power supply includes a filamenttransformer having its secondary winding connected to a filament cathodein said x-ray tube and having its primary winding connected to saidcontrol means.
 7. An x-ray apparatus comprising: an x-ray tube having ananode, at least one thermionic cathode and a stabilizer electrodepositioned behind the electron emitter of said cathode with respect tosaid anode; power supply means for applying high voltage between saidanode and cathode, and for applying a heating current to said cathode tocause electrons to be emitted from said cathode and to bombard saidanode as an electron discharge current; and stabilization meansincluding a first means separate from said stabilizer electrode fordetecting the amount of the electron discharge current flowing in saidanode, said first means applying a stabilizing signal voltage to saidstabilizer electrode to cause a portion of said electrons to be divertedfrom said anode to said stabilizer electrode depending upon the value ofsaid stabilizing voltage and said first means varying said stabilizingvoltage in accordance with any changes in the anode current in a mannerwhich maintains said anode current substantially constant.
 8. An x-rayapparatus in accordance with claim 7 in which the x-ray tube includes afocusing cup type control electrode separate from said stabilizerelectrode and connected to a pulser means for applying pulses to saidcontrol electrode of sufficient voltage to change the bias on the x-raytube to a conducting state from a quiescent nonconducting state.
 9. Anx-ray apparatus in accordance with claim 7 in which the first means iscoupled to a high voltage transformer of the power supply means by alight coupling means for transmitting a light signal corresponding tosaid stabilizing signal from said power supply means to said first meansof said stabilization means.
 10. An x-ray apparatus in accordance withclaim 7 in which the stabilization means also includes a second meansfor detecting the amount of current flowing in the stabilizer electrodeand for varying the cathode heating current of the x-ray tube tomaintain its anode current substantially constant, said first meansoperating faster than said second means, and said second means beingconnected so that it gradually takes over stabilization of the anodecurrent and reduces the stabilizing signal voltage so as to preventelectron heating of the stabilizer electrode.